This invention concerns a system and method for spacecraft autonomous recovery from Inertial Measurement Unit (IMU) failure events. During the failure event and recovery, the system maintains payload pointing to prevent any disruption to mission operations.
Three-axis zero momentum attitude control architectures have gained popularity as the size and complexity of communications spacecraft have grown. Such systems offer improved payload pointing and can accommodate a diverse range of payload antenna configurations. For a zero-momentum system, pointing control is provided by sensing three-axis attitude using attitude sensors such as earth sensors and sun sensors. Three-axis angular rates are typically measured by an Inertial Measurement Unit (IMU) that includes at least three gyros. Attitude control torques are applied using reaction wheels or thrusters in response to sensed attitude and rate errors to maintain earth pointing control. Typically, data from the earth sensor, which provides roll and pitch, is available continuously. Sun sensor data, which provides yaw information, is available intermittently. The angular rate data must be available continuously to maintain attitude control, as this data is used to propagate the three-axis inertial attitude and provide rate feedback information.
Typically, a spacecraft will include a redundancy management system that monitors the health of on-board equipment to detect and correct failure conditions. If a failure is detected, a backup or redundant component is activated to replace the function performed by the failed component. If an IMU failure occurs, the IMU is reconfigured to switch to a redundant power supply, processor, or gyro. During the period of the switch and immediately afterwards, valid angular rate data will not be available, and attitude control cannot be maintained using the normal three-axis zero-momentum approach.
Prior art systems use several methods to solve this problem. One approach is to xe2x80x9ccoastxe2x80x9d through the data interruption, by disabling the closed-loop attitude control system for the outage period. The drawback of this approach is that if data validity is not restored within some short interval, the spacecraft will lose earth-pointing control, thereby disrupting mission operations. Another approach is to use two IMUs that are both powered on, so that a xe2x80x9chotxe2x80x9d switch can be made from the failed to the redundant unit, without interrupting the flow of valid rate data to the control system. The disadvantage of this approach is that it requires an attitude control architecture with at least two entirely separate IMUs. Such an architecture is more costly and less mass-efficient than an architecture with a singe xe2x80x9cinternally redundantxe2x80x9d IMU.
The present invention provides a system that addresses shortcomings in known systems. The present invention provides a solution for IMU fault recovery that prevents any disruption in earth pointing, whether or not IMU failure correction is immediate. Furthermore, the system does not rely on having two or more separate IMU units.
The present invention provides a fault tolerant attitude control system for a zero momentum spacecraft. The system includes a zero momentum attitude control system operable to control spacecraft attitude utilizing data received from an earth sensor, a sun sensor, and the inertial measurement unit. The system also includes a gyroless attitude control system operable to control spacecraft attitude without receiving data from the inertial measurement unit. Additionally, the system includes a redundancy management system operable to monitor an inertial measurement unit to detect faults and to reconfigure the inertial measurement unit if a fault is detected and operable to determine when the inertial measurement unit failure is corrected and its data is again valid. Furthermore, the system includes a controller operable to automatically switch the spacecraft from the zero momentum attitude control to the gyroless attitude control when a fault in the inertial measurement unit is detected and to automatically switch the spacecraft from the gyroless attitude control to the zero momentum control upon resolution of the fault.
The present invention also includes a method for controlling attitude of a spacecraft. The method includes controlling the attitude of the spacecraft with a three-axis zero momentum attitude control system operable to control spacecraft attitude utilizing data received from an earth sensor, a sun sensor, and an inertial measurement unit. The inertial measurement unit is monitored to detect faults. The inertial measurement unit is reconfigured if a fault is detected. Attitude control of the spacecraft is switched from the zero momentum attitude control to the gyroless attitude control when a fault in the inertial measurement unit is detected. The gyroless attitude control system is operable to control spacecraft attitude without receiving data from the inertial measurement unit. Attitude control of the spacecraft is switched from the gyroless attitude control to the zero momentum attitude control when a fault in the inertial measurement unit is corrected and its data is again valid.
The present invention also includes a spacecraft that includes an earth sensor, a sun sensor, and an inertial measurement unit. A redundancy management system is operable to monitor the inertial measurement unit to detect faults and to reconfigure the inertial measurement unit if a fault is detected and operable to determine when the inertial measurement unit failure is corrected and its data again valid. A fault tolerant attitude control system includes a zero momentum attitude control system operable to control spacecraft attitude utilizing data received from the earth sensor, the sun sensor, and the inertial measurement unit, a gyroless attitude control system operable to control spacecraft attitude without receiving data from the inertial measurement unit. A controller is operable to automatically switch the spacecraft from the zero momentum attitude control to the gyroless attitude control when a fault in the inertial measurement unit is detected and to automatically switch the spacecraft from the gyroless attitude control to the zero momentum control upon resolution of the fault.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from a review of the following detailed description. The detailed description shows and describes preferred embodiments of the present invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, without departing from the present invention. Accordingly, the drawings and description are illustrative in nature and not restrictive.